For grinding ceramic materials, various types of super abrasive grinding wheels bonded by different kinds of materials are used. In this grinding operation, clogging occurs between wheel grains on the peripheral surface of the grinding wheel. Cutting action of the grains is reduced by the presence of removed stock between the grains, and the cutting edges of wheel grains are almost covered by removed stock. This is called clogging.
Clogging not only reduces grinding efficiency, but also generates minute vibrations of the wheel. This vibration is caused by two factors, i.e. resistance of the workpiece to be ground against the grinding action of the grinding wheel and the revolving movement of the grinding wheel, each of which affects the other element. Smoothly finished surfaces can not be achieved due to this minute vibration. Elimination of clogging is strongly desired in order to achieve long exposure of cutting edges of the super abrasive grains.
Conventional methods hitherto used to remove clogging, are:
(1) Use of a hand stick for dressing, which stick has a diamond grain, and
(2) Use of a crushing roll made of hardened tool steel or sintered abrasive grains.
The above methods have a common difficulty in that both methods remove clogging on the periphery of the grinding wheel and expose cutting edges of abrasive grains together with the bonding material which keeps the grains on the wheel surface. Especially, when the hand-stick type of tool is used, application of the tool on the grinding wheel requires a delicate touch and careful craftsmanship so as to not remove the wheel grains together with the clogging material.
There are other known dressing methods. These are:
(1) A method which ejects highly pressured (more than 100 kgf./cm.cm.) grinding lubricant on the wheel surface and blows off clogging material,
(2) A method which ejects highly pressurized grinding lubricant mixed with abrasive grains on the surface of the grinding wheel (Japanese Provisional Patent Application Sho59 (1984) - 219 158),
(3) A method which discharges grinding lubricant and abrasive grains separately between the grinding wheel and the crushing roll (Japanese Provisional Patent Application Sho54 (1979) - 141 487), and
(4) A method which blasts abrasive particles such as Alumina particles from a blasting gun by a highly pressurized (4 kgf./cm.cm.-6 kgf./cm.cm.) air stream, and generates cutting edges on abrasive grains of the grinding wheel.
The above methods which remove clogging of grinding wheels using pressurized liquid or a mixture of abrasive particles and liquid, need to have special apparatus to practice each method. The pressure used to cause removal of clogging material is rather high, and often removes the bonding material which keeps super abrasive grains on the surface of the grinding wheel. As a result, the bonding strength is weakened and usable abrasive grains drop from the wheel surface during grinding.
In another known method which uses air blasting of dry abrasive particles for dressing, the impact force of blasted abrasive particles can be adjusted by regulating the air pressure used as an accelerating force for the particles. But, in real practice, adjustment is difficult. If the pressure of the air is too high, usable grains on the wheel drop off. If the pressure of the air is too weak, dressing is not enough. Also, reclaiming blasted abrasive particles, and air circulation for re-use is difficult.
As mentioned above, conventional dressing methods and related apparatus can not suitably satisfy conditions demanded for dressing of grinding wheels.
Thus, the present invention offers a new dressing method for super abrasive grinding wheel which works steadily using low fluid pressure, and an apparatus suitable to practice this invention. More specifically, the invention relates to a method which uses gentle wet blasting for effectively dressing clogged grinding wheels.
To solve problems as mentioned in the above paragraphs, the dressing method of this invention uses a slurry containing 10% or less percentage of abrasive particles relative to the total volume of the slurry. This slurry is induced into a blasting gun which uses pressurized fluid as a source of blasting energy. The slurry is blasted by the gun against a surface of a super abrasive grinding wheel with an ejecting pressure of 2.0 to 3.5 kgf./cm.cm. and, in the blasting process, the liquid in the slurry is mistified and said mist accompanies the abrasive particles to assist in washing away the clogging material.
In the method described above, mist of liquid and abrasive particles are blasted against the periphery of a rotating super abrasive grinding wheel in such way that the direction of line of blasting is normal to the circumference of the grinding wheel or along a line deviating from normal within a few degrees thereof (i.e. from 0.degree. to 10.degree.) and toward the direction of turning of the revolving wheel.
In the apparatus embodying this invention, a slurry transportation device is provided in which slurry flowing down through an outlet located at the bottom of a slurry reservoir is connected with an upward opening of a three-opening connector. Another opening of the connector is for delivering slurry. A nozzle with a tapered inside hole is mounted detachably and exchangeably in this delivery opening. The remaining opening is in alignment with the delivery opening and is connected to an upper part of the slurry reservoir by bypass piping. An outlet of the bypass piping opens into a tapered nozzle hole defining the delivery opening so that liquid from the upper part of the slurry reservoir flows down through the bypass piping and is discharged into the tapered nozzle hole.
In the blasting gun, the mixture of liquid and abrasive particles (i.e. slurry is induced into the blasting gun, and is ejected with a highly pressurized fluid discharged from a jet part mounted in the blasting gun. The highly pressurized fluid may be supplied from a separate pressure intensifier apparatus which raises the pressure to such level that slurry is sucked and accelerated by a jet stream of the fluid when the fluid is discharged from the jet part of the blasting gun.
In the method of the present invention, slurry including abrasive particles and water in which abrasive particles occupies a maximum of 10% in volume ratio, is blasted to a super-abrasive grinding wheel with a blasting pressure at exit of the nozzle of 2.0 to 3.5 kgf./cm.cm. Blasted abrasive particles hit the stock removed by the grinding wheel and stuck between abrasive grains of the wheel, and remove said stock sticking between the wheel grains. Mistified water washes off said removed stock and eliminates clogging on the wheel surface.
Wear of bonding material existing between wheel grains can be lessened by setting the direction of the blasting stream of abrasive particles and mistified liquid along a line normal to the periphery of the wheel or along a line almost equal to the normal line but with a few degrees of deviation from said normal line, which deviation is opposed to the direction of wheel rotation.
In the device for transporting slurry, vacuum created at the mixing nozzle sucks liquid reserved in the slurry reservoir through bypass piping, and this liquid is sent to the gun. Flow generated at the mixing nozzle fluidizes accumulated grains at the joint section of the three way connector, and sends abrasive particles together with liquid to the blasting gun. In this mixing of abrasive particles and liquid, the ratio of solid particles and liquid in slurry can be adjusted to keep the volume ratio of solid particles to liquid no more than 10% of the total volume.
In the blasting gun, a pressure intensifier separate from the device can be used. This pressure intensifier uses another high pressure fluid as a driving force and pressurizes another liquid (water) above ambient pressure. Pressurized liquid is sent to the blasting gun, and sucks slurry. Blasting pressure at exit of the blasting nozzle can be controlled between 2.0 to 3.5 kgf.cm.cm. by regulating the pressure of fluid and adjusting the size of air port in the gun.